Interest in the field of opto-electronics has triggered an intense search for organic compounds (monomeric, polymeric or heterogeneous) which incorporate the desired optical properties (e.g., large optical nonlinearity, fast optical response, transparency to laser radiation, and phase matchability) into a robust material which can be easily fabricated. Although a variety of organic systems (e.g., crystalline compounds, inclusion complexes, and polymers) have been shown to be capable of second harmonic generation, many fewer systems have been developed for generating third order nonlinear optical effects.
It has now been found that J-aggregates are capable of producing resonant third order nonlinear optical effects, with .chi..sup.(3) values which rival those of any known organic system. Moreover, the J-aggregates can be formed in solution, or incorporated into glasses or polymers to give materials which can be fabricated into a variety of shapes.
Aggregation of certain organic dyes to form the so-called "J-aggregates" is a well-known, but little-understood, phenomenon that was first observed in the 1930's.
The absorption spectra of most dyes follow Beer's Law over a wide concentration range, so that the measured absorption of light is proportional to the concentration of the dye in solution. It has been observed for some dyes, however, that certain changes in the environment of the dye (e.g., temperature, concentration, dielectric constant) cause the appearance of a new absorption maximum at a wavelength longer than that of the monomer (i.e., the unperturbed transition obtained in highly dilute aqueous or alcoholic solutions, sometimes referred to as the M-band). The new transition, which is sometimes called a J-band, can be narrow, intense, and can exhibit resonance fluorescence. The appearance of J-bands has been rationalized in terms of dye aggregation. It is generally known that a J-aggregate consists of 100-500 dye monomers, but it's absorption spectrum stays unchanged once the size is longer than about 5 monomers. A comprehensive review of this subject can be found in A. H. Herz, Adv. in Colloid and Interface Science, 8, 237 (1977).
Dyes which form these so-called J-aggregates are generally large organic molecules (M.W. above 300) which have a small proportion of water-miscible to water-immiscible groups. Other molecular characteristics which favor the formation of J-aggregates include compactness and no large deviation from planarity. Classes of dyes which form J-aggregates include pyrilium, cyanine, carbocyanine, phthalocyanine and squaraine dyes. Some of these dyes are used as photosensitizers in photographic films.
J-aggregates of pyrilium dyes in certain polymeric matrices are known to be capable of second harmonic generation (U.S. Pat. No. 4,692,636). Third order nonlinear optical effects were not disclosed.
Nonresonant third order nonlinear polarizability of a number of linear conjugated molecules, including monomers, but not J-aggregates, of some cyanine dyes, has been disclosed by Stevenson et al. (Mat. Res. Soc Symp. Proc., Vol. 109, "Nonlinear Optical Properties of Polymers", edited by A. J. Heeger, et al., pp 103-108). Resonant third order nonlinear effects were not disclosed, nor would they be anticipated on the basis of the nonresonant properties.
Moosad et al., Pramana 1988, Vol. 31, No. 4, pp 281-287, have disclosed the properties of a few saturable absorbers as candidates for low power optical phase conjugation (OPC). The specific systems studied were the dyes eosin, erythrosin B, and Rose Bengal, doped in gelatin, poly(vinyl alcohol) and boric acid glass films. However, these systems do not exist in J-aggregate form and, consequently, do not exhibit enhanced third order optical nonlinearity.